The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube having a high resolution and an external size suited for use as a display monitor of the desktop size.
The color cathode ray tube is frequently employed as a TV receiver and as a display monitor for various information terminals, such as personal computers or workstations. In particular, the cathode ray tube for a color display monitor of the so-called "desktop terminal" type, such as a personal computer to be mounted on a desk for use by a user, is required to have a display performance of high resolution on a large screen and to be mounted compactly in a restricted space.
At present, the typical large size color cathode ray tube to be used as a display monitor of a desktop terminal of this kind is the 17 type (the effective display area of which has a diagonal size of about 41 cm) and the 21 type (with a diagonal size of about 51 cm). The 17 type color display monitor has a display performance of 1.3 M pixels. For a higher resolution, the 21 type color display monitor is used (which has a display performance of 2 M pixels: a horizontal dot number of not less than 1,450/line, preferably 1,600/line).
From the point of view of size, the 21 type color display cathode ray tube has an overall length of more than 450 mm, so that the length of the display monitor exceeds 500 mm. On the other hand, the typical table or desk has a top which is as deep as about 700 mm. If the depth of the keyboard is 150 mm, and if the area for accommodating the hands of the user is as deep as 100 mm, the usability of the display monitor becomes undesirable, if it is to be employed in a desktop size terminal.
If the display resolution of 2 M pixels is to be realized in the screen size of a 20 type color display monitor (corresponding to a 20 type color cathode ray tube having a panel unit with a diagonal diameter of 52 cm) or less, on the other hand, the hole pitch of the shadow mask must be reduced, which causes a problem in that the allowance for adjusting the color purity is deteriorated. In order to compensate for this, the transmittance of the holes of the shadow mask has to be lowered, which causes a drop in the brightness of the color cathode ray tube. Moreover, the color irregularity at the central portion and the peripheral portion of the screen becomes so serious as to cause a practical problem in the image quality.
Thus, a display monitor employing the color cathode ray tube of the prior art does not have a resolution corresponding to 2 M pixels and is not able to meet the size requirements necessary for desktop use.
In this kind of color cathode ray tube, on the other hand, a glass material forming the panel unit of the color cathode ray tube is a so-called "tinted material" or "dark tinted material" for improving the contrast and preventing the reflection of incoming light. In addition, the glass panel unit of the color cathode ray tube of the prior art is thinner at the central portion of its effective display area and thicker at the corner portion thereof, and this thickness difference is 1.7 mm or more. This makes a luminance difference due to the optical transmittance difference of the glass between the central portion and the corner portion irregular, causing a deterioration in the image quality.